1. Field of the Invention
The invention relates to a shuttle kiln for firing ceramic porous bodies containing organic binders, and more specifically to a shuttle kiln suitable for firing ceramic honeycomb structures.
2. Description of Related Art
As a furnace for firing ceramic porous bodies to be mass-produced, a continuous-type furnace represented by a tunnel kiln or a roller hearth kiln and a batch-type furnace represented by a shuttle kiln are widely used. The shuttle kiln is a batch-type firing furnace for firing porous materials loaded on a carriage stored in the furnace and is named so because the carriage reciprocates between the inside and the outside of the furnace.
In firing of ceramic porous bodies containing organic binders in the shuttle kiln, in an early phase of temperature rising, a large amount of organic binder vapor generates from the unfired ceramic porous bodies. To secure safety in this phase, it is necessary to control the concentration of the organic binder vapor in the furnace to a level below the explosion limit. Typical examples of organic binder are methylcellulose, polyvinyl alcohol, etc.
Typically, a burner is ignited at high excess air ratio m of 30 to 50, and its burner flame speed is increased to stir the inside of the furnace efficiently and also introduce a large amount of air into the furnace in order to control the concentration of the organic binder vapor to a level below the explosion limit. Therefore, naturally, an oxygen concentration in the furnace rises to burn the organic binder vapor at the surface of the ceramic porous bodies. However, since ceramic porous bodies such as a ceramic honeycomb structure have excellent thermal insulation properties, the following problems occur.
That is, in the early phase, as the organic binder vapor burns, a periphery of the ceramic porous body is heated to promote releasing of binder. However, the heat of the ceramic porous body is hardly transferred to the inside because of its excellent thermal insulation properties. Then, as the internal temperature rises gradually, the organic binder in the furnace burns progressively so that the internal temperature of the ceramic porous body may start rising. However, the heat does not easily transfer to the periphery of the ceramic porous body, and its inside temperature becomes higher than periphery. Consequently, tensile force acts inside the ceramic porous body in the early phase of a binder releasing process and, by contrast, acts outside in the later phase of the process. As a result, the ceramic porous body is liable to have cracks referred to as “breaks” and therefore be defective.
To avoid such problem, the rate of temperature rise in the furnace can be decreased to slow down the binder releasing process. At the same time, however, the firing cycle is prolonged, inevitably resulting in decrease in productivity.
To solve the problem, Patent Document 1 according to the application by the present applicant discloses a technology of decreasing the concentration of oxygen in the furnace in the binder releasing process to thereby suppress burning of organic binders so that the cracks occurrence may be prevented. As specific means for the purpose, a method of reducing the air ratio of a burner and a method of introducing a nitrogen gas is described.
In the case of a ceramic honeycomb structure, to suppress a temperature difference that causes the cracks, the oxygen concentration should be kept at 8% or less, or preferably 5% or less. However, this is accompanied by cost-related problems because, for example, a large amount of nitrogen gas is required. Further, if the burner's air ratio is decreased to lower the in-furnace oxygen concentration, stirring in the furnace cannot become well due to an insufficient amount of the internal gas and its internal temperature distribution is liable to be inhomogeneous.
Patent Document 2 describes another method to lower the in-furnace oxygen concentration by use of a large amount of low-oxygen gas, specifically by suctioning an in-furnace gas containing organic binders gas, completely burning it with an afterburner, and drawing back the generated combustion gas into the furnace.
However, in the case of a batch-type furnace represented by a shuttle kiln, a suction port for the in-furnace gas is exposed to a high temperature of 1000° C. or higher during a firing process following the binder releasing process and cannot be sealed completely; therefore, not a small amount of fresh air is mixed when the in-furnace gas is suctioned. Moreover, in the case of a shuttle kiln, as a carriage needs to reciprocate between the inside and the outside of the furnace, if the in-furnace suction port is formed at the lower part of the furnace body, namely in the carriage, it is very difficult to seal a portion between the port and a smoke path inlet port located at a lower part of the carriage. Therefore, fresh air is mixed inevitably when the in-furnace gas is suctioned. As a result, the oxygen concentration of combustion gas generated via the afterburner decreases only to about 10%, causing a problem that even if the combustion gas is drawn back into the furnace, the oxygen concentration in the furnace cannot be kept at a sufficiently low level.